Securing a computer is not a simple process, so I will describe the steps involved  and provide links to more detailed instructions and tutorials. 

 1. Change the default password. The suggested Raspbian distro includes this as an option in the initial startup script. If you did not do this already you can use the [passwd][1] command to change it.
 2. [Disable unused services][2]. I saw that the recommended Raspbian distro includes the Apache web server and enables it at startup (can someone confirm this). Do you really need a web server running? If not disable it. 
 3. install and configure [iptables][3].
 4. Keep your system up to date. You can automate this using [cron][4] or using [cron-apt][5].
 5. [Configure logging][6] to monitor logins and failed login attempts. If possible use an external Hard drive to host you /var partition, this will give you more space, avoid the log files from filling up the SD Card and extend the life of your SD Card. 

This is only the bare minimum steps for securing your Pi. For more info you may want to read the [Securing Debian Manual][7]. 


  [1]: http://www.computerhope.com/unix/upasswor.htm
  [2]: http://theos.in/desktop-linux/removing-unwanted-startup-debian-files-or-services/
  [3]: http://wiki.debian.org/iptables
  [4]: http://www.debian-administration.org/articles/56
  [5]: http://www.debianadmin.com/automatic-update-of-packages-using-cron-apt.html
  [6]: http://wiki.kartbuilding.net/index.php/Debian_logs
  [7]: http://www.debian.org/doc/manuals/securing-debian-howto/